The laptop should now be able to boot from external media, such as SD-cards and USB-pens, when pressing the Ctrl and U keys. In addition to that, it will also be able to boot images that haven't been signed.

Partitioning

The following GPT partition lay-out is recommended:

1st partition: ChromeOS kernel partition.

2nd partition: root partition.

In order to set up the partition lay-out, one can use cgpt or gdisk. Here we'll be using cgpt:

Then we can bundle up the kernel and the device tree into a FIT-image. Download kernel.its and save it as kernel.its in chromeos/arch/arm/boot (in the kernel repository you cloned before). After you have saved the kernel.its file, you can bundle up the kernel and device tree:

mkimage -f arch/arm/boot/kernel.its vmlinux.uimg

The FIT-image then has to be signed using the developer keys. The vbutil_kernel utility will do that for us. In addition to signing the image, it is also possible to add a kernel cmdline to the image. We'll be using /proc/cmdline as a template:

cp /proc/cmdline kernel-cmdline

Open up kernel-cmdline, and change the rootfs arguments to: "root=/dev/mmcblk1p2 rootfstype=ext4 rootwait rw"